Impact of fixed orthodontic treatment with and without premolar extraction on gingival display index in men and women

Fixing a gummy smile is a crucial esthetic treatment. It is unclear if the modality of orthodontic treatment (i.e., premolar extraction versus non-extraction) can affect the extent of gingival show. There is a lack of literature on this topic. Therefore, this retrospective 2-year study was conducted. This retrospective cohort study was performed on 468 observations of 234 patients. Fixed orthodontic treatment of 234 patients was performed using 22-MBT edgewise applications during a period of 1.5 to 2 years. Of these, 117 patients had extraction treatment protocols and 117 had non-extraction ones. Gingival displays of patients were measured on their smile photographs, once before beginning the orthodontic treatment and once after finishing the treatment. The index of gingival display was calculated as the ratio of the gingiva-to-tooth heights. It was calculated by measuring the extent of gingival appearance of each maxillary anterior tooth, averaging these six measurements, and dividing this average appearance by the clinical height of the right maxillary central. These values were analyzed using repeated-measures two-way ANOVA and paired and unpaired t-tests (α = 0.05, α = 0.025). There were 56 and 59 females in the extraction and non-extraction groups, respectively (chi-square, P = 0.794). In the non-extraction group, Gingival Show Index reduced from 50.2 to 39.7% (P = 0.0000, paired t-test), while in the extraction group, it increased from 20.1 to 26.8% (P = 0.0371, t-test). The repeated-measures two-way ANOVA showed that the effects of treatment modalities (non-extraction group showing greater gingival display, P = 0.000) and sexes (women showing greater gingival displays, P = 0.001) were significant. The interaction of treatment by sex was significant (P = 0.045). The test detected different effects for both treatment, since the interaction of treatment type with the repeated-measures factor 'time' was significant (P = 0.0000). However, the interaction of the repeated-measures factor 'time' with gender was not significant (P = 0.587). The interaction of the factors 'time by treatment by sex' was significant (P = 0.034). Premolar extraction can increase the gingival display index for about 7%, while non-extraction treatment can reduce gingival show ratio for about 11%. Women have a greater percent of gingival display index (both before and after the treatment) compared to men.